The Difference Between Mist and Fog

The morning after the Dartmoor talk, I took the opportunity to do a little natural navigation exercise on the moor.

There was heavy rain and plenty of mist and fog. I’ll be writing more about the natural navigation exercise itself in my next newsletter (you can subscribe at the bottom of the page), but here I’ll stick to explaining the difference between mist and fog.

Both mist and fog are created when the air becomes saturated and water vapour condenses to form droplets that hang in the air. Air becomes saturated more quickly at lower temperatures, so mists and fogs are more likely when a) the air is very humid and/or b) the air is relatively cool.

Mist and fog are both, in physical terms, the same as clouds. The difference between clouds and mist or fog, is just in the eye of the beholder. Clouds you see hugging a mountaintop will be mist or fog to a walker near that summit.

The difference between mist and fog is a simple one and it is about degrees of visibility. It does no harm to think of fog as just dense mist, or if you prefer, mist as a light fog. But if you are being precise and, depending on the authority you defer to, there is a specific difference between mist and fog.

The most common practical definition is this:

In mist visibility is reduced, but we can see 1km or further.

In fog visibility is reduced below 1km.

So, it is not at all uncommon for a fog to clear a little to become a mist, which might later thicken back into a fog again. All the time this is happening, those in the valleys below are looking up and commenting on the pretty way the clouds are hugging the hills.

In the image above we are looking at fog. The Dartmoor triangulation pillar is visible, but it is less than 100 metres away. Visibility is probably between 100m and 200m, making it a fog and quite a thick one at that.